As is well known, a regulator for an automotive alternator is mainly to drive the excitation winding of the alternator such that a regular voltage value is obtained at the output of a voltage generator for charging the battery and, hence, power the electric utilities of the whole automobile vehicle.
The battery charging procedure depends on the type of the battery and its construction technology.
The specifications usually set for the voltage regulator of an alternator provide for:
its output voltage to lie close to a reference value; PA1 the regulator switching frequency to be within a predetermined frequency range.
A typical precision value requirement is of 300 mV for steady-state error. Also, minimum and maximum frequencies would be set at 50 Hz and 400 Hz, respectively.
These specifications must be held for a load variation within 10% to 90% of the maximum current, and a variation of the alternator speed within 2,000 to 20,000 rpm.
In addition, the appearance of a noise signal at the alternator output, as represented by a sinusoidal function, and particularly a rectified sinus, makes these specifications quite strict. In fact, the peak-to-peak amplitude of this noise signal may be of 8-10V, and the noisy frequency range may extend from 1,200 Hz to 14,400 Hz.
Prior approaches have been aimed at reducing the amplitude of the noise signal by filtering the output voltage of the alternator through a linear filter.
In particular, a cutoff frequency of -3 dB is selected for the linear filter, as a tradeoff between an adequate filtering of the noise signal and the assurance of a minimum switching frequency of the alternator under light load conditions.
In actual practice, this tradeoff is never fully successful in filtering out the noise signal, and an additional digital filter, called a stretcher, has to be provided.
In particular, the error due to the difference between the output voltage and the reference voltage is filtered by the linear filter, and then "squared" through a comparator in order to produce a square wave, called the field control signal.
The signal applied to the comparator input is less than thoroughly filtered by the linear filter, and due to the presence of the noise signal, it is bound to produce spurious comparator switchings. The field control signal output from the comparator is input to the stretcher filter.
The stretcher filter FA, shown schematically in FIG. 1, will present, at its output terminal OUT correspondingly with a square-wave input signal S1 having a period T and duty cycle D being applied to its input terminal IN, a square-wave output signal S2 having the period T and a duty cycle (DT+t.sub.A)/T, where t.sub.A is a parameter referred to as the delay of the stretcher filter FA.
Shown in FIG. 2 on a common time base t, are the patterns of the input S1 and output S2 square wave signals of the stretcher filter FA of FIG. 1.
The stretcher filter FA suppresses most of the spurious switchings, thereby reducing the switching frequency of the linear filter/stretcher filter/comparator system.
While being advantageous in several ways, this prior approach cannot suppress all of the comparator spurious switchings, and requires a stretcher filter with a non-trivial delay t.sub.A.
In addition, the provision of a stretcher filter is apt to enhance the steady-state error, which is also dependent on the load being applied to the alternator and on the engine revolutions per minute.
What is needed is a voltage regulator for an alternator, which has such constructional and functional features as to ensure regulation of the switching frequency of the output control signal and accuracy of the alternator steady state, thereby overcoming the limitations with which prior art regulators have been beset.
What is also needed is a frequency filtering process of a fuzzy and non-linear type.